In traditional photography, it is customary to determine the light capturing ability of a film, or its photographic speed, through the method defined in ISO 5800 "Color Negative Films for Still Photography-Determination of ISO Speed". The method contained therein ensures that films exposed to the calculated amount of light will have the manufacturer's intended contrast over the entire scene luminance range. Films with the ability to be exposed with lower amounts of light are referred to as "faster" and have higher calculated speed values; those requiring more light are referred to as "slower" and have lower calculated speed. For each doubling of the requisite light, the calculated speed is decreased by one half. A film that has one-half the ISO calculated speed of a second film is said to be "one stop" slower than the second film; a film that has one-fourth the calculated speed of a second film is said to be "two stops" slower than the second film.
The method defined in ISO 5800 ensures that exposures made with one-half the intended amount of light will also have acceptable contrast in the resulting print when printed under the same conditions. This so-called underexposure latitude is required because of error in exposure determination by cameras and exposure meters, and because optical printing methods for color negative films do not allow for contrast adjustment of individual exposures. When a negative is exposed to less than one-half the intended amount of light, the film is underexposed, and the resulting print has low contrast; that is, there is not sufficient density range in the negative to produce a print with the desired density range.
The ability to record images with low amounts of light is of increasing importance to consumers. In many situations, the use of artificial lighting, such as floodlight or flashbulbs is not permitted or practical. Moreover, as ISO speed increases, print graininess increases, manufacturing cost increases, and other characteristics, such as color intensity, color accuracy and sharpness generally decrease.
Consequently, a need exists for a photographic method that allows for a low cost, high quality film to be exposed at higher than the intended ISO speed and yield acceptable print results.